FIG. 3 shows a conventional thin-film thermal printhead disclosed in the Patent Document 1 described below. The thermal printhead B includes an insulting substrate 101, a heat-retaining glaze layer 102, a resistor layer 103 in the form of a thin film, an electrode layer 104 in the form of a thin film, and a protective layer 105. The heat-retaining glaze layer 102 is formed on the insulating substrate 101 and includes a bulging portion 102c. The resistor layer 103 is formed as a thin film on the heat-retaining glaze layer 102 by sputtering and covers the bulging portion 102c. The electrode layer 104 is formed as a thin film on the resistor layer 103 by sputtering and divided by an electrode layer gap 104c positioned on the top of the bulging portion 102c. The protective layer 105 covers both of the resistor layer 103 and the electrode layer 104. Due to the presence of the electrode layer gap 104c, the resistor layer 103 includes a portion which is not covered by the electrode layer 104, i.e., a heating portion 107. With this arrangement, when current flows through the electrode layer 104, Joule heat is generated at the heating portion 107. The thermosensitive recording or thermal transfer recording is performed by utilizing this heat.
The electrode layer 104 comprises a single layer having a uniform thickness of at least 0.5 μm and generally about 0.8 μm. The electrode layer 104 includes an electrode pad having a predetermined thickness so that a metal wire is reliably bonded to the electrode pad in wire bonding.
However, to secure a sufficient thickness of the electrode layer 104 causes the following drawbacks.
Firstly, a stepped portion 104d having a height of at least 0.5 μm which corresponds to the thickness of the electrode layer is formed at the end of the electrode layer 104 which adjoins the heating portion 107. Due to the stepped portion 104d, a stepped portion 105d is formed at the protective layer 105 laminated on the electrode layer. The stepped portion 105d hinders the close contact between the thermal printhead B and a recording medium, and hence, hinders proper utilization of the heat generated by the heating portion 107 for printing. Further, when foreign matter enters the space between the thermal printhead B and the recording medium, the foreign matter may be caught in the stepped portion 105d. In such a case, the protective layer may be damaged or peeled off.
Secondly, the heat generated at the heating portion 107 is likely to escape through the thick electrode layer 104. Thus, the heat generated by the heating portion 107 is not utilized effectively.
Thirdly, as shown in FIG. 4, when the thickness of the electrode layer 104 exceeds 0.5 μM, small projections 108 called hillocks are formed on the surface of the electrode layer 104 due to the growth of Al crystal. Due to the hillocks 108, small projections 109 are formed on the surface of the electrode layer 104. The projections 109 increase the coefficient of friction between the protective layer 105 and the printing medium, and hence, cause meandering or clogging of the printing medium. Further, due to the contact of the recording medium with the projections 109, an excessively large external force is applied to the projections 109. As a result, the projections 109 or the protective layer 105 may be broken. In such a case, ions such as Cl− or Na+ enter through the broken portion, so that the electrode layer 104 is corroded.
Patent Document 1: JP-A-2001-105641